1. Field of the Invention
The invention generally relates to a process for forming metal packages in a continuous package formation/filling operation and particularly to a process for forming such a metal package in situ with a coating thereon, particularly a coating formed of an organic plastic material.
2. Description of the Prior Art
According to the present practices in the mass production of packaging, pharmaceuticals, foodstuffs and other products are packaged in integrated packaging processes which operate at a high production rate, the individual packages being produced in situ continuously from sheets of plastics material. The packages are thus produced in an economical manner immediately before filling of the packages, thereby avoiding wasteful handling and storage. In prior packaging processes of this type, organic plastic materials have been fabricated by thermoforming techniques. Products which are particularly sensitive to oxidation or to light, however, cannot be packaged adequately through the use of plastic, metal packaging being required to ensure proper sealing. It has not been possible previously to utilize the in situ thermoforming packaging processes referred to above in situations where metal packaging was required since it has not been possible to fabricate packages of metal, even of aluminum, through use of a thermoforming process. Metal packaging has therefore typically been formed by operation separate from the package filling operation.
A thermoforming process comprises the heating of a thin-walled blank, in most cases a cup-shaped member or a simple flat sheet, to an elevated temperature which is lower than the melting temperature of the material which is to be molded but which is sufficient to soften that material to a "plastic" state. The desired shape is then imparted to the blank by applying the blank against the surface of a mold by the action of a fluid under pressure. At the present time, thermoforming has been used to mold various metals such as alloys based on magnesium, aluminum, copper, titanium, stainless steel and nickel. Aluminum alloys known as superplastic alloys have been developed which are particularly suited to use in thermoforming processes. However, thermoforming can only be used with metals to produce slow rates of deformation of the metal, French Pat. No. 2,004,410, for example, indicating that the removal of thin-walled members in a hot condition from a mold gives rise to serious difficulties. The operation of removing formed metal members from a mold may be facilitated by coating the molds with a foundry wash (clay and resin) or by coating the surface of the blank with a graphite oil. Such techniques require cleaning after package formation, particularly when foodstuffs are to be packaged. These supplementary operations increase costs and reduce the speed of production.
For the reasons described, thermoforming of metal has been restricted hitherto to small-scale production of complicated members such as those for the aviation industry or for data processing equipment. Prior to the present invention, the production of metal containers by thermoforming at industrial rates of production and integration of such a production process into a continuous packaging train has been considered impossible. Thus, at the present time, metal containers are produced in a cold condition by processes such as stamping or rolling of a sleeve-like member followed by welding and crimping of the members. The metal surfaces of such packages then must be cleaned and pickled before coated with varnish or lacquer. Such a mode of manufacture involves operations both of a chemical and of a mechanical nature, the operations being carried out at varying rates thereby rendering it virtually impossible to integrate all or even most of the operation into a continuous process for packaging consumable products. The metal containers are at present produced in specialist factories from which they must be dispatched to the packaging location, thereby giving rise to the necessity for costly intermediate storage. All these handling operations, interruptions in loading feed, and storage, increase the cost of the metal containers which, due to the cost of the material itself, is already higher than the cost of containers made of standard plastic material. Finally, if such containers are to be used for aseptic filling, they must be sterilized before filling.